"Biological anthropologist Rebecca Ackermann of the University of Cape Town in South Africa co-organized the session to introduce researchers steeped in human evolution to the ins and outs of hybridization in animals and its potential for helping to identify signs of interbreeding on fossils typically regarded as either H. sapiens or Neandertals.

“'I was astonished by the number of people who came up to me after the session and said that they hadn’t even thought about this issue before,” Ackermann says.

"Interbreeding is no rare event. Genome comparisons have uncovered unexpectedly high levels of hybridization among related species of fungi, plants, rodents, birds, bears and baboons, to name a few. Species often don’t fit the traditional concept of populations that exist in a reproductive vacuum, where mating happens only between card-carrying species members.

"Evolutionary biologists increasingly view species that have diverged from a common ancestor within the last few million years as being biologically alike enough to interbreed successfully and evolve as interconnected populations. These cross-species collaborations break from the metaphor of an evolutionary tree sprouting species on separate branches. Think instead of a braided stream, with related species flowing into and out of genetic exchanges, while still retaining their own distinctive looks and behaviors.

"Research now suggests that hybridization sometimes ignites helpful evolutionary changes. An initial round of interbreeding — followed by hybrid offspring mating among themselves and with members of parent species — can result in animals with a far greater array of physical traits than observed in either original species. Physical variety in a population provides fuel for natural selection, the process by which individuals with genetic traits best suited to their environment tend to survive longer and produce more offspring.

"Working in concert with natural selection and random genetic changes over time, hybridization influences evolution in other ways as well. Depending on available resources and climate shifts, among other factors, interbreeding may stimulate the merger of previously separate species or, conversely, prompt one of those species to die out while another carries on. The birth of new species also becomes possible. In hybrid zones where the ranges of related species overlap, interbreeding regularly occurs.

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"However genes, behaviors and beliefs got divvied up in the Stone Age, a mix of regional populations — including Neandertals and Denisovans — can be considered human ancestors, she theorizes. They all contributed to human evolution’s braided stream.

"That’s a controversial view. Neandertals and Denisovans lived in relatively isolated areas where contact with other hominid populations was probably rare, says paleoanthropologist Matthew Tocheri of Lakehead University in Thunder Bay, Canada. Random DNA alterations, leading to the spread of genes that happened to promote survival in specific environments, played far more important roles in human evolution than occasional hybridization did, Tocheri predicts.

"Neandertals and Denisovans can’t yet boast of being undisputed hybrid powers behind humankind’s rise. But a gallery of interbreeding animals could well help detect hybrid hominids hiding in plain sight in the fossil record."